Systems and methods for selectively activating virtual guide geometries

ABSTRACT

A method for selectively activating a virtual geometry includes establishing a plurality of virtual geometries, each virtual geometry having a target feature and being available to be activated to guide an instrument to the target feature by restricting movement of the instrument within the confines of the virtual geometry. The method further includes displaying the plurality of virtual geometries on a display to a user and receiving user input selecting a virtual geometry from the displayed plurality of virtual geometries. The virtual geometry is activated based on the selection.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/340,668 filed Dec. 29, 2011, which is incorporated by referenceherein in its entirety.

BACKGROUND

The present invention relates generally to haptic guidance systems and,more particularly, to systems and methods for selectively activatinghaptic guide zones.

Many surgical procedures require the use of specialized tools to performsurgical tasks with a high degree of accuracy and precision. In somecases, such surgical procedures require precise positioning and/orplacement of the tool at or near a particular point within a patient'sanatomy. For example, many orthopedic procedures rely on the accurateplacement of pins, screws, guide and/or post holes, or other elements ina precise position and orientation with respect to an anatomical featureof the patient. In order to ensure that these elements are properlypositioned and oriented, great care is required on the part of thesurgeon to ensure that the surgical tool(s) (e.g., drill, saw, reamer,etc.) used to position these elements is precisely and accuratelyaligned with the anatomy of the patient. However, this can beparticularly challenging without the use of a guide and even morechallenging in minimally-invasive procedures where visibility at thesurgical site is limited or, in some cases, nonexistent.

Early solutions for enhancing the accuracy and precision of thealignment of tools in a surgical environment involved the use ofmechanical guide elements, such as jigs. These mechanical guides weretypically placed and/or mounted in close proximity to the anatomy of thepatient and provided a physical guide that maintained a desired positionand orientation of the tool during its operation.

For example, some prosthetic implants used in knee joint replacementsurgeries comprise projections, keels, and/or other mechanical elementsthat are configured to fit within corresponding holes or voids createdin the bone to secure the implant to the bone. In order to ensure theaccurate placement of these voids, a jig was often used to mechanicallyalign a drill in a desired position and orientation with respect to thebone of a patient. During operation of the drill, the jig would maintainthe desired orientation while the surgeon advanced the drill into to thebone until the desired depth was reached.

Although these guide jigs enhanced the accuracy and precision of theplacement of voids within the bone, they needed to be physicallyinstalled in proximity to the bone during the surgical procedure. Theaccurate alignment and placement of these guides can take a considerableamount of time, which could prolong the surgical procedure. Furthermore,mechanical jigs and cutting guides are typically too large to fit withinthe relatively small spaces allowed for minimally-invasive procedures.

With the advent of computer-assisted surgery (CAS) systems, surgeonswere no longer required to rely on mechanical jigs for precisionpositioning of surgical instruments. Specifically, many CAS systemsinclude surgical navigation and tracking software that displays agraphical representation of the surgical site. Using the navigation andtracking features of the CAS system, the surgeon can view the locationof a surgical instrument relative to the patient's anatomy. Using thegraphical interface as a guide, the surgeon can manually navigate thesurgical tool to a desired position within the surgical site.

More sophisticated CAS systems are configured for interactive couplingwith the surgical tools. These CAS systems may be equipped with forcefeedback controls that provide the surgeon with haptic feedback when,for example, the surgical tool interacts with certain pre-establishedvirtual boundaries. Such virtual boundaries may be established toconstrain the surgical instrument from undesired interactions withcertain areas of the patient's anatomy. By strategically arranging thevirtual boundaries for the force feedback controls, users can create“virtual” guides that define the areas in which the tool can operate, aswell as areas that prohibit tool operation. If a surgical procedurerequires the drilling of a post hole in a patient's bone, a virtualboundary may be established to define the desired position, orientation,and size of the hole. The virtual boundary may constrain a surgical toolfrom operating outside of the established boundary.

Although existing virtual guide methods provide a solution for definingthe areas of allowed operation (and corresponding areas of constrainedoperation) of a surgical instrument, they may still be inefficient. Forexample, conventional virtual guide methods do include a solution foraligning a surgical tool in a proper orientation prior to engagementwith the patient's anatomy. As a result, in surgical procedures thatrequire precision cuts having specific orientations (such as thedrilling of post or guide holes within bone), the surgeon may berequired to manually “search” for the appropriate orientation by usingthe tip of the surgical tool as an exploring device to first locate theengagement point at the surface of the patient's bone. Once theengagement point has been located, the surgeon then manually pivots thesurgical tool to locate the appropriate orientation for advancing thetool to the target point. Not only is such a manual process frustratingto the surgeon, it may unnecessarily prolong the surgery, which canincrease costs.

Moreover, existing CAS systems may not provide an effective solution forenabling and disabling haptic zones during the performance of a surgicalprocedure. This is particularly problematic in situations in whichmultiple haptic boundaries are located in close proximity with (oroverlap) one another. In such situations, the haptic boundaries mayprovide conflicting haptic feedback, constraining movement of thesurgical instrument in an undesirable or unintended manner. For example,in situations in which haptic zones overlap, a first haptic zone mayconstrain movement of the surgical instrument in one direction, while asecond haptic zone may constrain movement of the surgical instrument inthe opposite direction. As a result, movement of the surgical instrumentmay be severely limited by the conflicting haptic feedback imposed onthe instrument by the first and second haptic zones. It may therefore beadvantageous to provide a solution for selectively or sequentiallyactivating individual haptic guide zones to limit the possibility ofconflicts between overlapping haptic zones.

The presently disclosed systems and methods for selectively activatinghaptic guide zones are directed to overcoming one or more of theproblems set forth above and/or other problems in the art.

SUMMARY OF THE INVENTION

According to one aspect, the present disclosure is directed to a methodfor activating a virtual haptic geometry based on a position of aportion of an instrument relative to a target point. The methodcomprises detecting a presence of a reference point of an instrumentwithin a threshold distance of a target point. A virtual haptic geometrycorresponding to the target point may be activated in response to thedetected presence of the reference point of the instrument within thethreshold distance.

In accordance with another aspect, the present disclosure is directed toa computer-implemented method for activating a virtual haptic geometry.The method may include determining, by a processor associated with acomputer, a position of a reference point of an instrument. A distancebetween the reference point and each of a plurality of target points maybe determined by the processor. The method may also include identifying,by the processor, the target point that is closest to the referencepoint, and activating a virtual haptic geometry associated with theidentified target point.

According to another aspect, the present disclosure is directed to amethod for activating a virtual haptic geometry. The method may includeidentifying a plurality of open target points associated with a surgicalenvironment, and determining a position of a reference point of asurgical instrument relative to each of the identified open targetpoints. The method may include identifying a first open target pointthat is closest to the reference point, and activating a virtual hapticgeometry associated with the first open target point that is closest tothe reference point. A position of the reference point may be monitoredand, after determining that the reference poi.at has reached the firstopen target point, the virtual haptic geometry may be deactivated.

In accordance with yet another aspect, the present disclosure isdirected to a computer-assisted surgery system comprising a surgicalinstrument for performing at least one task associated with a surgicalprocedure and a processor, operatively coupled to the surgicalinstrument. The processor may be configured to detect a presence of areference point of the surgical instrument within a threshold distanceof a target point. A virtual haptic geometry corresponding to the targetpoint may be activated based on the detected presence of the referencepoint of the instrument.

According to yet another aspect, the present disclosure is directed tocomputer-implemented method for activating a virtual haptic geometry.The method may also comprise displaying, by a processor on a displayassociated with a computer, a plurality of target points. A virtualhaptic geometry associated with one target point of the plurality oftarget points may be activated based, at least in part, on a userselection of the one target point from among the plurality of targetpoints.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a perspective view of an exemplary surgical environmentin which systems and methods consistent with the disclosed embodimentsmay be employed;

FIG. 2 provides a schematic illustration of an exemplarycomputer-assisted surgery (CAS) system, in which certain methodsconsistent with the disclosed embodiments may be implemented;

FIG. 3 provides a schematic diagram of an exemplary computer system,which may be used in one or more components associated with the CASsystem illustrated in FIG. 2;

FIG. 4 provides an illustration of an exemplary virtual haptic volume,consistent with certain disclosed embodiments;

FIG. 5 provides a 2-dimensional side view of a virtual haptic volume,consistent with the disclosed embodiments;

FIG. 6 provides a side view of an exemplary surgical environment havinga plurality of target points, which may be selectively activated usingthe systems and methods consistent with certain disclosed embodiments;

FIG. 7 provides a flowchart depicting an exemplary method for activatinga haptic guide zone, consistent with certain disclosed embodiments; and

FIG. 8 provides a flowchart depicting an exemplary method forselectively activating a haptic guide zone, consistent with thedisclosed embodiments.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments of thepresent disclosure, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or similarparts.

FIG. 1 illustrates an exemplary surgical environment 100 in whichprocesses and methods consistent with the disclosed embodiments may beemployed. As illustrated in FIG. 1, many surgical procedures, such asknee replacement procedures, require accurate and precise modificationto the patient's anatomy. One such example is the placement of post orguide holes 102 a-102 c within a patient's tibia 101 using a surgicalinstrument, such as drill (not shown), having a rotary cutting tool 103.Because these post or guide holes 102 a-102 c correspond to projectionson a prefabricated prosthetic implant (not shown), each hole should beaccurately and precisely placed at a specific location, depth, andorientation within the patient's bone.

In order to ensure efficient and proper alignment of the post holeswithin the patient's anatomy, a computer-aided surgery (CAS) system maybe used to generate a graphical representation of the surgical site anda corresponding virtual guide that may aid the surgeon in properlyaligning the tool prior to interaction with patient's anatomy. The CASsystem consistent with the present disclosure may also provide a hapticfeedback geometry that captures the surgical tool while the toolapproaches the engagement site. Once captured, the boundaries of thevirtual haptic geometry may limit or restrict the movement of thesurgical instrument within the confines of a haptic volume defined bythe virtual haptic geometry. Based on the surgeon's movements, thehaptic volume may be gradually reduced, limiting the range of motion ofthe surgical instrument until the surgical tool is aligned with thetarget access associated with the post holes. Systems and methods foraligning the surgical instrument consistent with the disclosedembodiments are discussed in greater detail below and in theaccompanying drawings.

As explained, many CAS systems include software that allows users toelectronically register certain anatomic features (e.g., bones, softtissues, etc.), surgical instruments, and other landmarks associatedwith the surgical site. CAS systems may generate a graphicalrepresentation of the surgical site based on the registration of theanatomic features. The CAS software also allows users to plan certainaspects of the surgical procedure, and register these aspects fordisplay with the graphical representation of the surgical site. Forexample, in a knee joint replacement procedure, a surgeon may registertarget navigation points, the location and depth of bone and tissuecuts, virtual boundaries that may be associated with a correspondingreference for the application of haptic force, and other aspects of thesurgery.

FIG. 2 provides a schematic diagram of an exemplary computer-assistedsurgery (CAS) system 200, in which processes and features associatedwith celtain disclosed embodiments may be implemented. CAS system 200may be configured to perform a wide variety of orthopedic surgicalprocedures such as, for example, partial or total joint replacementsurgeries. As illustrated in FIG. 2, CAS system 200 may comprise atracking system 201, computer-assisted navigation system 202, one ormore display devices 203 a, 203 b, and a robotic arm 204. It should beappreciated that CAS system 200, as well as the methods and processesdescribed herein, may be applicable to many different types of jointreplacement procedures. Although certain disclosed embodiments may bedescribed with respect to knee replacement procedures, the concepts andmethods described herein may be applicable to other types of orthopedicsurgeries, such as partial hip replacement, full or partial hipresurfacing, shoulder replacement or resurfacing procedures, and othertypes of orthopedic procedures.

Robotic arm 204 can be used in an interactive manner by a surgeon toperform a surgical procedure, such as a knee replacement procedure, on apatient. As shown in FIG. 2, robotic arm 204 includes a base 205, anarticulated arm 206, a force system (not shown), and a controller (notshown). A surgical tool 210 (e.g., an end effector having an operatingmember, such as a saw, reamer, or burr) may be coupled to thearticulated arm 206. The surgeon can manipulate the surgical tool 210 bygrasping and manually moving the articulated arm 206 and/or the surgicaltool 210.

The force system and controller are configured to provide control orguidance to the surgeon during manipulation of the surgical tool. Theforce system is configured to provide at least some force to thesurgical tool via the articulated arm 206, and the controller isprogrammed to generate control signals for control ling the forcesystem. In one embodiment, the force system includes actuators and abackdriveable transmission that provide haptic (or force) feedback toconstrain or inhibit the surgeon from manually moving the surgical toolbeyond predefined virtual boundaries defined by haptic objects asdescribed, for example, in U.S. Pat. No. 8,010,180 and/or U.S. patentapplication Ser. No. 12/654,519 (U.S. Patent Application Pub. No.2010/0170362), filed Dec. 22, 2009, each of which is hereby incorporatedby reference herein in its entirety. According to one embodiment, CASsystem 200 is the RIO® Robotic Arm Interactive Orthopedic Systemmanufactured by MAKO Surgical Corp. of Fort Lauderdale Fla. The forcesystem and controller may be housed within the robotic arm 204.

Tracking system 201 may include any suitable device or system configuredto track the relative locations, positions, orientations, and/or posesof the surgical tool 210 (coupled to robotic arm 204) and/or positionsof registered portions of a patient's anatomy, such as bones. Suchdevices may employ optical, mechanical, or electromagnetic pose trackingtechnologies. According to one embodiment, tracking system 201 maycomprise a vision-based pose tracking technology, wherein an opticaldetector, such as a camera or infrared sensor, is configured todetermine the position of one or more optical transponders (not shown).Based on the position of the optical transponders, tracking system 201may capture the pose (i.e., the position and orientation) information ofa portion of the patient's anatomy that is registered to thattransponder or set of transponders.

Navigation system 202 may be communicatively coupled to tracking system201 and may be configured to receive tracking data from tracking system201. Based on the received tracking data, navigation system 202 maydetermine the position and orientation associated with one or moreregistered features of the surgical environment, such as surgical tool210 or portions of the patient's anatomy. Navigation system 202 may alsoinclude surgical planning and surgical assistance software that may beused by a surgeon or surgical support staff during the surgicalprocedure. For example, during a joint replacement procedure, navigationsystem 202 may display images related to the surgical procedure on oneor both of the display devices 203 a, 203 b.

Navigation system 202 (and/or one or more constituent components of CASsystem 200) may include or embody a processor-based system (such as ageneral or special-purpose computer) in which processes and methodsconsistent with the disclosed embodiments may be implemented. Forexample, as illustrated in FIG. 3, CAS system 200 may include one ormore hardware and/or software components configured to execute softwareprograms, such as, tracking software, surgical navigation software, 3-Dbone modeling or imaging software, and/or software for establishing andmodifying virtual haptic boundaries for use with a force system toprovide haptic feedback to surgical tool 210. For example, CAS system200 may include one or more hardware components such as, for example, acentral processing unit (CPU) (processor 231); computer-readable media,such as a random access memory (RAM) module 232, a read-only memory(ROM) module 233, and a storage device 234; a database 235; one or moreinput/output (I/O) devices 236; and a network interface 237. Thecomputer system associated with CAS system 200 may include additional,fewer, and/or different components than those listed above. It isunderstood that the components listed above are exemplary only and notintended to be limiting.

Processor 231 may include one or more microprocessors, each configuredto execute instructions and process data to perform one or morefunctions associated with CAS system 200. As illustrated in FIG. 2,processor 231 may be communicatively coupled to RAM 232, ROM 233,storage device 234, database 235, I/O devices 236, and network interface237. Processor 231 may be configured to execute sequences of computerprogram instructions to perform various processes, which will bedescribed in detail below. The computer program instructions may beloaded into RAM for execution by processor 231.

Computer-readable media, such as RAM 232, ROM 233, and storage device234, may be configured to store computer-readable instructions that,when executed by processor 231, may cause CAS system 200 or one or moreconstituent components, such as navigation system 202, to performfunctions or tasks associated with CAS system 200. For example, computerreadable media may include instructions for causing the CAS system 200to perform one or more methods for determining changes in parameters ofa hip joint after a hip arthroplasty procedure. Computer-readable mediamay also contain instructions that cause tracking system 201 to capturepositions of a plurality of anatomical landmarks associated with certainregistered objects, such as surgical tool 210 or portions of a patient'sanatomy, and cause navigation system 202 to generate virtualrepresentations of the registered objects for display on I/O devices236. Exemplary methods for which computer-readable media may containinstructions will be described in greater detail below. It iscontemplated that each portion of a method described herein may havecorresponding instructions stored in computer-readable media for causingone or more components of CAS system 200 to perform the methoddescribed.

I/O devices 236 may include one or more components configured tocommunicate information with a user associated with CAS system 200. Forexample, I/O devices 236 may include a console with an integratedkeyboard and mouse to allow a user (e.g., a surgeon) to input parameters(e.g., surgeon commands 250) associated with CAS system 200. I/O devices236 may also include a display, such as monitors 203 a, 203 b, includinga graphical user interface (GUI) for outputting information on amonitor. I/O devices 236 may also include peripheral devices such as,for example, a printer for printing information associated with CASsystem 236, a user-accessible disk drive (e.g., a USB port, a floppy,CD-ROM, or DVD-ROM drive, etc.) to allow a user to input data stored ona portable media device, a microphone, a speaker system, or any othersuitable type of interface device. For example, I/O devices 236 mayinclude an electronic interface that allows a user to input patientcomputed tomography (CT) data 260 into CAS system 200. This CT data maythen be used to generate and manipulate virtual representations ofportions of the patient's anatomy (e.g., bones) in software.

Processor 231 associated with CAS system 200 may be configured toestablish a virtual haptic geometry associated with or relative to oneor more features of a patient's anatomy. As explained, CAS system 200may be configured to create a virtual representation of a surgical sitethat includes, for example, virtual representations of a patient'sanatomy, a surgical instrument to be used during a surgical procedure, aprobe tool for registering other objects within the surgical site, andany other such object associated with a surgical site.

In addition to physical objects, CAS system 200 may be configured togenerate virtual objects—objects that exist in software, and which maybe useful during the performance of a surgical procedure. For example,CAS system 200 may be configured to generate virtual boundaries thatcorrespond to a surgeon's plan for preparing a bone, such as boundariesdefining areas of the bone that the surgeon plans to cut, remove, orotherwise alter. Alternatively or additionally, CAS system 200 maydefine virtual objects that correspond to a desired path or course overwhich a portion of surgical tool 210 should navigate to perform aparticular task.

According to one embodiment, CAS system 200 may be configured togenerate a virtual haptic geometry that defines a point, line, surface,or volume in a virtual coordinate space. The virtual haptic geometry maybe associated with a haptic feedback or force system of CAS system 200such that, when a tracked position of a portion of the surgical tool(e.g., an established center point 103 a or tool axis 403 of cuttingtool 103) interacts with the virtual haptic geometry, a haptic feedbackis generated and applied to surgical tool 210. FIGS. 4 and 5 providealternate views of an exemplary virtual haptic geometry 400 that may begenerated consistent with the presently disclosed embodiments.

According to one exemplary embodiment, and as illustrated in FIG. 4,virtual haptic geometry 400 may be a substantially funnel-shaped volumethat is positioned and oriented relative to a patient's anatomy, such asfemur 101. As such, virtual haptic geometry 400 may define a virtualpathway to quickly and efficiently guide and position a surgicalinstrument, such as rotary drill or burr, into a proper alignmentrelative to femur 101 prior to engagement with femur 101. According tothe embodiment illustrated in FIG. 4, virtual haptic geometry maycomprise a substantially cone-shaped portion that converges toward asubstantially cylindrically-shaped portion. The cylindrically-shapedportion may, extend toward a target end point (402 in FIG. 5), which, inthe example illustrated in FIG. 4, corresponds to the depth of postholes 102.

FIG. 5 illustrates a side view of the exemplary virtual haptic geometry400 shown in FIG. 4, being accessed by cutting tool 103. As illustratedin FIG. 5, virtual haptic geometry 400 may be defined about a targetaxis 401 that includes a target end point 402. Virtual haptic geometry400 may comprise a boundary surface 404, which may be positioned at aninitial boundary angle, θ_(B), relative to the target axis 401.According to the embodiment shown in FIG. 5, boundary surface 404 maydefine a substantially cone-shaped volume having an initial base radius,r. It should be noted, however, that, although the upper portion ofvirtual haptic geometry 400 is illustrated in certain embodiments ashaving a substantially cone-shaped boundary surface, it is contemplatedthat virtual haptic geometry 400 may include or embody any shapesuitable for guiding a cutting tool 103 toward a target end point 402.For example, as shown in FIG. 8, boundary surface 404 may define asubstantially curved upper portion, which is designed to converge towardthe target axis more aggressively than the substantially linear boundarysurface shown in FIG. 5.

Target end point 402 may be a user-defined point that corresponds to thetarget destination of at least a portion of cutting tool 103. Accordingto one embodiment, target end point 402 may define a target depth 410 ofpost or guide hole 102 of femur 101. As such, target end point 402corresponds to a desired depth 410 that a reference point 103 a of tool103 (also referred to herein as a tool center point (TCP)) can reachbefore a haptic feedback force is applied to surgical tool 210.

Target axis 401 may include target end point 402 and may serve as acentral axis about which virtual haptic geometry 400 may be defined.Target axis 401 may also define a desired axis for approaching targetend point 402 with cutting tool 103. As such, target axis 401 may definethe axis to which virtual haptic geometry 400 converges, and maycorrespond to the desired or ideal orientation of approach of surgicaltool 403 toward target end point 402.

During operation of CAS system 200 and in accordance with an exemplaryembodiment, virtual haptic geometry 400 becomes associated with cuttingtool 103 when reference point 103 a of cutting tool 103 enters thevolume defined by virtual haptic geometry. Once active, virtual hapticgeometry 400 may be configured to provide a haptic feedback when cuttingtool 103 interacts with one or more virtual boundaries 404. For example,virtual haptic geometry 400 may define a haptic “wall” that constrains,inhibits, or prevents cutting tool 103 and/or reference point 103 a frommoving beyond the boundary surface 404 of virtual haptic surface 400. Inan exemplary embodiment, virtual haptic geometry 400 may be configuredwith an “opening” for allowing cutting tool 103 to disengage fromvirtual haptic geometry 400. While this disengagement opening may belocated anywhere along virtual haptic geometry 400, an exemplaryembodiment includes the opening along the surface of virtual hapticgeometry 404 that is located farthest from bone or tissue surfaces ofthe patient. In the embodiment illustrated in FIG. 5, the top surface ofboundary surface 404 may be configured as the disengagement opening.

As an alternative or in addition to constraining the movement of cuttingtool 103 (and/or tool reference point 103 a) to within the volumedefined by virtual haptic geometry 400, CAS system 200 may be configuredto guide cutting tool 103 toward a desired orientation prior toengagement with femur 101. Specifically, CAS system 200 may beconfigured to monitor a tool orientation angle, θ_(T), which comprisesthe angle between the tool axis 403 and a target axis 401. As will beexplained in further detail below, CAS system 200 may be configured tofacilitate alignment of the tool axis 403 with target axis 401 bymodifying the location of boundary surface 404 of virtual hapticgeometry 400 based on the location of tool axis 403.

To ensure that cutting tool is positioned in the proper orientationprior to engagement with a surface of the patient's anatomy, anintermediate haptic threshold may be established and associated withvirtual haptic boundary 400. Specifically, the virtual haptic boundary400 may include an intermediate tool stop haptic plane 420 that provideshaptic feedback if reference point 103 a attempts to advance withoutbeing in the proper orientation. The haptic feedback may include ahaptic wall that constrains advancement of cutting tool 103 pastintermediate tool stop haptic plane 420 if one or more of tool axis 403and/or tool reference point 103 a is not aligned with target axis 401.

As explained, software associated with CAS system 200 may be configuredto register and track certain aspects of surgical environment 100. Forexample, cutting tool 103, along with other features of surgicalenvironment 100, may be registered and associated with a virtualcoordinate space for tracking and display by CAS system 200. As such,tracking system 201 of CAS system 200 can determine the relativeposition and orientation of cutting tool 103 in the virtual coordinatespace.

In order to properly monitor the orientation of tool axis 403 of cuttingtool 103, the tool axis 403 of cutting tool 103 should first beregistered for tracking in virtual coordinate space of CAS system 200.According to one embodiment (such as that illustrated in FIG. 5), toolaxis 403 may correspond to the central axis of a rotary burr, whichpasses through reference point 103 a associated with the center of thetip of the rotary burr. Tool axis 403, along with reference point 103 a,may be determined during a calibration process of CAS system 200, priorto the surgical procedure. Alternatively or additionally, CAS system 200may be registered as part of the registration process during thesurgical procedure by using a pre-calibrated registration probe tocapture and record the locations of a plurality of points along toolaxis 403. Because the position of the axis at the surface of thecylindrically-shaped rotary burr is slightly different than the positionof the axis at the true center of tool axis 403, CAS system 200 may beprovided with an offset to project the tool axis 403 to the center ofcutting tool 103.

According to yet another embodiment, the pre-calibrated registrationprobe may be used to capture a large number of points along the surfaceof cutting tool 103. Based on the relative locations of these points,software associated with CAS system 200 may be configured to derive toolaxis 403. It is contemplated that additional and/or different methodsmay be used for registering various aspects of cutting tool 103 thanthose that are listed above. For example, a virtual software modelrepresenting cutting tool 103 may be generated using computed tomography(CT) scan information. The model may be registered in the virtualcoordinate space using the calibrated registration probe. Onceregistered, tracking system 201 associated with CAS system 200 canmonitor the real-time location, position, and orientation of registeredcomponents of surgical environment 100 relative to the establishedvirtual coordinate space in order to guide cutting tool 103 to, targetend points 402 a-402 c by sequentially activating each of target endpoints 402 a-402 c in accordance with the processes and methodsconsistent with the disclosed embodiments.

FIG. 6 illustrates a surgical environment 100 having a plurality oftarget end points 402 a-402 c, each of which is associated with arespective virtual haptic geometry 400 a-400 c for guiding a cuttingtool 103 toward target end points 402 a-402 c, respectively. Asillustrated in FIG. 6, portions of virtual haptic geometries 400 a-400 cmay overlap with one another. If each of virtual haptic geometries 400a-400 c are “active” at the same time, haptic forces associated with onevirtual haptic geometry 400 a may interfere with one or more of hapticforces associated virtual haptic geometries 400 b, 400 c, particularlyif cutting tool 103 is located in one of the overlapping regionsassociated with virtual haptic geometries 400 a-400 c. As such,processes consistent with the disclosed embodiments provide a solutionfor selectively activating one of virtual haptic geometries 400 a-400 cat a time.

FIG. 6 illustrates an embodiment in which a first virtual hapticgeometry 400 a is selectively “activated,” while the remaining virtualhaptic geometries 400 b, 400 c are maintained in an inactive state. Assuch, only haptic forces associated with active virtual haptic geometry400 a are operable as the haptic guide zone for guiding reference point103 a of cutting tool 103 toward target end point 402 a associated withactive virtual haptic geometry 400 a. The remaining haptic zones areselectively and/or sequentially activated once the task associated withvirtual haptic geometry 400 a is completed, unless and until the activevirtual haptic geometry 400 a is deactivated.

According to one embodiment, virtual haptic geometry 400 a isautomatically activated when cutting tool 103 (and/or reference point103 a) is positioned within a threshold distance of target end point 402a associated with virtual haptic geometry 400 a. For example, softwareassociated with CAS system 200 may be configured to track the positionof a reference point 103 a of cutting tool 103 relative to target endpoints 402 a-402 c. CAS system 200 may determine which of target endpoint 402 a-402 c is closest to reference point 103 a of cutting tool103, and activate the corresponding virtual haptic geometry 400 aassociated with the closest target end point 402 a.

It is contemplated that additional and/or different criteria may be usedfor selectively activating virtual haptic geometries 400 a-400 c. Forexample, CAS system 200 may be configured to selectively activatevirtual haptic geometries 400 a-400 c when reference point 103 a ofcutting tool 103 enters a volume associated with one of virtual hapticgeometries 400 a-400 c. Alternatively or additionally, CAS system 200may be configured to selectively activate one or more of virtual hapticgeometries 400 a-400 c when reference point 103 a is within thethreshold engagement area associated with one of virtual hapticgeometries 400 a-400 c. The engagement area may be defined by anengagement height, h, (measured from a surface of tibia 101 to apredetermined height threshold 610) and an engagement distance, d,(measured from target axis 401 a to a predetermined distance threshold620).

According to one exemplary embodiment, CAS system 200 activates virtualhaptic geometry 400 a when reference point 103 a of cutting tool 103 iswithin both (1) the volume defined by virtual haptic geometry 400 a and(2) the threshold engagement area. By providing multiple criteria forselectively activating one virtual haptic geometry (e.g., virtual hapticgeometry 400 a) from among a plurality of virtual baptic geometries 400a-400 c, CAS system 200 may be configured to prevent “accidental”activation of a virtual haptic geometry 400 a-400 c that can result fromusing a single criterion for selectively activating virtual hapticgeometries 400 a-400 c.

As an alternative or in addition to selectively activating virtualhaptic geometry 400 a based on a position of cutting tool 103 relativeto target end point 402 a, virtual haptic geometries 400 a-400 c may beactivated manually, based on a user selection using a graphical userinterface associated with CAS system 200. That is, a user of CAS system200 may selectively activate virtual haptic geometry 400 a from amongavailable or “open” virtual haptic geometries 400 a-400 c. According toone embodiment, the manual activation technique can be implemented inaddition to the automatic activation technique described above. Forexample, CAS system 200 may be configured to automatically activatevirtual haptic boundaries 400 a-400 c based on the tracked position ofreference point 103 a of cutting tool 103, as described above. However,the manual activation technique may be available to the user on anad-hoc basis, allowing a user of CAS system 200 to selectively manuallyoverride the automatic activation technique.

Processes and methods consistent with the disclosed embodiments providea solution for quickly and efficiently guiding cutting tool 103 to aproper orientation for engagement with a patient's anatomy. Exemplarymethods consistent with the disclosed embodiments track the position andorientation of tool axis 403 relative to a target axis 401. As theorientation angle, θ_(T), between tool axis 403 and target axis 401becomes smaller, virtual haptic geometry 400 associated with surgicaltool 210 is repositioned behind the virtual representation of cuttingtool 103, creating a boundary surface that “collapses” as cutting tool103 is brought into alignment with target axis 401. As a result, cuttingtool 103 is constrained from being rotated to an angle that is greaterthan the smallest orientation angle. This “collapsing” virtual hapticgeometry effectively allows the surgeon the freedom to move or rotatecutting tool 103 only to those positions that bring cutting tool 103 incloser alignment with target axis 401. Eventually, as the surgeonrotates cutting tool 103 within the area defined by virtual hapticgeometry 400, the boundary decreases in size until tool axis 403 issufficiently aligned with target axis 401.

In addition, processes and methods consistent with the disclosedembodiments provide a solution for selectively (and sequentially)activating haptic guide zones to limit the potential for interferencebetween adjacent or overlapping haptic guide zones. Accordingly, systemsand methods consistent with certain disclosed embodiments provide asolution for selectively activating haptic guides zones based on theproximity of cutting tool 103 to a target point (such as target endpoint 402 a-402 c or an engagement point 605 a-605 c). In accordancewith an exemplary embodiment, haptic zones that are not activated remaininactive. Consequently, corresponding haptic forces associated withinactive haptic zones will be disabled and prevented from operating oncutting tool 103.

Alternatively or additionally, certain processes and methods consistentwith the disclosed embodiments provide a feature that allows a user toselect, via a graphical user interface, one haptic zone from a pluralityof available haptic zones that is to be activated. The unselected hapticzones may remain inactive, at least until the task associated with thehaptic zone has been completed or until the user otherwise deactivatesthe virtual haptic geometry (by, for example, exiting the virtual hapticgeometry).

FIG. 7 provides a flowchart 700 illustrating an exemplary method forselectively activating a virtual haptic geometry based on the positionof cutting tool 103 relative to surgical environment 100. As illustratedin FIG. 7, the process may involve establishing a plurality of virtualhaptic geometries 400 a-400 c for aiding in the guidance of cutting tool103 (and/or reference point 103 a associated therewith) to targetpoint(s) 402 a-402 c (Step 710). For example, during a planning phase ofsurgical procedure that involves the preparation of a bone for receivinga prosthetic implant, a user of CAS system 200 may plan the areas of thebone that require resection. In some situations, the implant includes aplurality of stabilizing projections (not shown) that are designed tofit within corresponding post holes created in the bone. During thesurgical planning process, the user of CAS system 200 may plan thelocation, depth, and orientation of post holes 102 a-102 c that are tobe resected in order to receive the corresponding projections of theimplant. The user may also specify target end point(s) 402 a-402 c,which defines the location that reference point 103 a of cutting tool103 should reach to meet the target depth parameter. The user may alsospecify target axes 401 a-401 c, which define the proper approachorientation of cutting tool 103 to achieve the desired orientation ofrespective post holes 102 a-102 c.

Once the target end point(s) 402 a-402 c and target axes 401 a-401 chave been created, software associated with CAS system 200 may establisha plurality of virtual haptic geometries 400 a-400 c. According to oneembodiment, virtual haptic geometries 400 a-400 c may each embodyfunnel-shaped haptic guide zones, such as that shown in FIG. 5. It iscontemplated, however, that additional and/or different sizes and shapesof haptic geometries may be used without departing from the scope of thepresent disclosure.

During operation, CAS system 200 may be configured to identify targetpoints 402 a-402 c (or other aspect of virtual haptic geometries 400a-400 c, such as bone surface engagement points 605 a-605 c) anddetermine the position of reference point 103 a of cutting tool 103(Step 720). Specifically, CAS system 200 may identify the position of apredetermined feature of each of virtual haptic geometries 400 a-400 cas a basis for comparison with the position of reference point 103 a.This predetermined feature may be any feature associated with virtualhaptic geometries 400 a-400 c that can be used as a reference fordetermining which of virtual haptic geometries 400 a-400 c is closest toreference point 103 a of cutting tool 103. For example, thepredetermined feature may comprise target points 402 a-402 c, engagementpoints 605 a-605 c, or a particular point on boundary surface 404 ofeach of virtual haptic geometries 400 a-400 c.

Upon identifying the plurality of planned target points associated withsurgical environment 100, the distance from reference point 103 a ofcutting tool 103 to each target point may he determined (Step 730).According to one embodiment, tracking system 201 of CAS system 200 maydetermine the position of reference point 103 a of a surgical instrument(e.g., cutting tool 103) within surgical environment 100. Softwareassociated with CAS system 200 may be configured to calculate thedistance between the position of reference point 103 a and each targetend point 402 a-402 c. Alternatively or additionally, softwareassociated with CAS system 200 may be configured to determine thedistance between reference point 103 a and other aspects of surgicalenvironment 100, such as target engagement points 605 a-605 c.Accordingly, “target point,” as the term is used herein, refers to anyfeature or point associated with surgical environment 100 that may betargeted for engagement by cutting tool 103, such as, for example,target engagement points 605 a-605 c and target end points 402 a-402 c.

Once the distance from reference point 103 a to each target point hasbeen determined, the target point that is closest to reference point 103a may be identified (Step 740). Specifically, software associated withCAS system 200 may compare the distances between reference point 103 aand each of the target points to identify the target point that isclosest to reference point 103 a. Based on the identified target point,CAS system 200 may activate the respective haptic geometry (e.g.,virtual haptic geometry 400 a of FIG. 6) associated with the identifiedtarget point (Step 750). The remaining virtual haptic geometries (e.g.,virtual haptic geometries 400 b, 400 c of FIG. 6) may remain inactivewhile virtual haptic geometry 400 a is active. According to theexemplary embodiment, only haptic forces associated with “active”virtual haptic geometries can operate to constrain the position andorientation of cutting tool 103.

It is contemplated that, although certain embodiments are described asusing distance between reference point 103 a and target points 402 a-402c as the basis for determining which of virtual haptic geometries 400a-400 c is activated, other features may be used for selectivelyactivating virtual haptic geometries 400 a-400 c. For example, CASsystem 200 may be configured to detem line the position of referencepoint 103 a of cutting tool 103 relative to an engagement area (definedby a height, h, above the surface of bone 101 and a distance, d, fromthe target axis 401) associated with each post hole. If the position ofreference point 103 a enters the engagement area corresponding to aparticular post hole, the virtual haptic geometry 400 a-400 c associatedwith the engagement area may be activated.

FIG. 8 provides a flowchart 800 illustrating an exemplary embodiment forsequentially activating virtual haptic geometries associated withsurgical environment 100. As illustrated in FIG. 8, the method mayinclude monitoring a position of reference point 103 a of cutting tool103 (Step 810). According to the exemplary embodiments described above,tracking system 201 of CAS system 200 may be configured to monitor theposition of reference point 103 a relative to one or more otherregistered components of surgical environment within the virtualcoordinate space.

While tracking the position of tool reference point 103 a, CAS system200 may be configured to determine whether tool reference point 103 a iswithin a threshold distance of a target point associated with one of theavailable or “open” virtual haptic geometries 400 a-400 c (Step 820).According to one embodiment, virtual haptic geometries 400 a-400 c maybe designated as “open” if the surgical plan has not yet been completedfor the respective virtual haptic geometry. Turning to FIG. 6, forexample, each of virtual haptic geometries 400 a-400 c may be designatedas “open” until the reference point 103 a of cutting tool 100 hasresected all of the bone within the volume defined by the respectivevirtual haptic geometry.

Once the resected bone has been removed and the post hole has beencompleted, CAS system 200 may “close” the virtual haptic geometry anddeactivate the haptic boundary associated therewith. CAS system 200 isconfigured to estimate the completion of the bone resection process bytracking the location of reference position 103 a of cutting tool 103relative to the bone and the respective virtual haptic geometry 400a-400 c associated therewith. For example, CAS system 200 may determinethat the bone resection operation for a first post hole is complete whenreference point 103 a of cutting tool 103 has successfully reachedtarget end point 402 a, which corresponds to the bottom of post hole 102a.

If CAS system 200 determines that tool reference point 103 a is notwithin a threshold distance of a target point associated with an “open”virtual haptic geometry (Step 820: No), the process may revert back tostep 810, whereby CAS system 200 continues to monitor the position oftool reference point 103 a. If, on the other hand, CAS system 200determines that reference point 103 a is within a threshold distance ofan “open” target point (Step 820: Yes), CAS system 200 may activate therespective virtual haptic geometry associated with the correspondingtarget point (Step 830). CAS system 200 may deactivate the remainingvirtual haptic geometries (and/or haptic forces associated therewith),so that haptic forces associated with the remaining geometries do notinterfere with haptic forces corresponding to the active virtual hapticgeometry.

Once the virtual haptic geometry is activated and tool reference point103 a is located therewithin, CAS system 200 may be configured to detemline whether tool reference position 103 a is trying to “break” out ofthe haptic volume defined by the virtual haptic geometry (Step 835).According to one embodiment, this involves tracking the position of toolreference point 103 a within the volume defined by virtual hapticgeometry 400. The tracked position of reference point 103 a may hecompared with the position of boundary surface 404. Tracking system 201of CAS system 200 may be configured to determine when the position ofreference point 103 a interferes, intersects, or otherwise overlaps withboundary surface 404. As the position of reference point 103 ainterferes with the position of boundary surface 404, CAS system 200 maybe configured to detect that reference point 103 a is trying to “break”through the haptic boundary defined by virtual haptic geometry 400.

If tracking system 201 associated with CAS system 200 determines thatreference point 103 a is trying to “break” through boundary surface 404associated with virtual haptic geometry 400 (Step 835: Yes), forcesystem of CAS system 200 may be configured to apply corresponding hapticforces to constrain the tool reference point within the haptic volume(Step 840). According to one embodiment, CAS system 200 is configured asan impedance-type haptic system, whereby haptic forces are designed tosimulate a mechanical impedance based on the position of reference point103 a relative to boundary surface 404 of virtual haptic geometry 400.It is contemplated, however, that, that processes and methods consistentwith the disclosed embodiments are also compatible with admittance-typehaptic force control systems. CAS system 200 may continue monitoring theposition of the tool reference point 103 a (Step 855) and revert back tostep 835.

If, however, tool reference point 103 a is not trying to break out ofthe volume defined by virtual haptic geometry (Step 835: No), CAS systemis programmed to monitor the progress of reference point 103 a ofcutting tool 103 and determine whether the reference point 103 a hasreached the target end point (Step 850). If reference point 103 a hasnot reached target end point (Step 850: No), the process may continue tomonitor the position of tool reference point 103 a (Step 855) and revertback to step 835.

After reference point 103 a has reached target point (Step 850: Yes),CAS system may determine when the reference point has been safelyextracted from the bone engagement surface (Step 860:Yes) beforedeactivating and closing the haptic guide zone (Step 880). Specifically,CAS system 200 may determine when the position of reference point 103 aof cutting tool 103 has been moved outside of an engagement threshold(not shown). This engagement threshold may embody a user-defined (orpredetermined) virtual boundary that is used as a trigger fordeactivating and closing a virtual haptic geometry upon completion ofbone resection process. To ensure that virtual haptic geometry remainsactive after the post hole is complete in order to help guide cuttingtool 103 while it is being extracted from the hole, the engagementthreshold may be established as a distance from the surface of the bone.According to one embodiment, this distance may be similar to that usedto define intermediate tool stop plane (shown as 420 of FIG. 5).

If reference point 103 a has not been extracted outside of thepredetermined engagement area (or distance) (Step 860: No), CAS system200 may continue monitoring the position of tool reference point 103 a(Step 870) and revert back to step 860. If, however, reference point 103a been extracted outside of the predetermined engagement threshold (Step860:Yes), CAS system 200 may deactivate the virtual haptic geometry (andhaptic forces associated therewith) and “close” the post hole fromfurther access by cutting tool 103 (Step 880). The process may thenrevert back to step 810, to continue selective activation of theremaining (i.e., “open”) virtual haptic boundaries.

As explained above, there are many alternatives for determining which ofvirtual haptic geometries should be activated based on the position ofreference point 103 a. For example, as an alternative or in addition todetermining whether tool reference point 103 a is within a thresholddistance of an “open” target point associated with a virtual hapticgeometry, CAS system 200 may be configured to determine whether toolreference point 103 a is within a threshold engagement area (or volume)associated with a virtual haptic geometry.

Alternatively, a combination of criteria may be used to determine whichof virtual haptic geometries 400 a-400 c is to be activated. Forexample, CAS system 200 may be configured to require that referencepoint 103 a (1) be located within the volume defined by a virtual hapticgeometry and (2) be within the threshold engagement area associated withthe post hole. FIG. 6 illustrates virtual haptic geometry 400 a, whichhas been activated using this multiple-criteria approach for activatingvirtual haptic geometries. By using multiple criteria for activatingvirtual haptic geometries, CAS system 200 may be configured to avoid“accidental” activation of haptic boundaries, which is more likely tooccur where only a single criterion is used.

It is also contemplated that CAS system 200 may include a solution formanually overriding the activation of virtual haptic geometries.According to one embodiment, this manual override feature may allow theuser to selectively activate (and deactivate) the virtual hapticgeometries based on the specific needs/preferences of the surgeon. Thus,an activated virtual haptic geometry 400 a may be manually deactivated,allowing the surgeon to selectively activate one of the other virtualhaptic geometries 400 b, 400 c. It should be noted that virtual hapticgeometry 400 a may remain “open” after the manual override, unlesscutting tool 103 has completed the resection of the bone associated withvirtual haptic geometry 400 a. It should also be noted that the manualactivate/deactivation feature may be disabled in certain situations,such as when reference point 103 a is located within bone (and/or thecylindrical portion of viltual haptic geometry 400 a) or when referencepoint 103 a is located within a predetermined distance of the surface ofbone 101.

Although virtual haptic geometry is illustrated and described in theexemplary embodiments as being a substantially funnel-shaped boundaryfor use in guiding a surgical drill or burr to a target point, thepresently disclosed embodiments are applicable for generating virtualhaptic boundaries having different shapes for use with other types oftools. For example, the presently disclosed embodiments contemplategenerating a collapsible virtual haptic geometry having a substantially“Y”-shaped cross section having a substantially “V”-shaped upper sectionthat converges to a substantially planar lower section for guiding aplanar or sagittal saw toward a desired orientation for executing aplanar cut on a bone. Consequently, it is contemplated that the methodsdescribed herein may be employed in virtually any environment where itmay be advantageous to guide a surgical tool to a predeterminedorientation for approaching a target tool operation site.

The presently disclosed systems and methods provide a solution thatenables a computer-assisted surgical system to sequentially and/orselectively activate virtual haptic guide zones, thereby ensuring thatoverlapping haptic guide zones do not interfere with one another duringoperation of cutting tool 103. More specifically, certain systems andmethods described herein implement a process that determines theposition of a reference point 103 a of a surgical instrument and, basedon this position, automatically activates the nearest available virtualhaptic guide zone. Once one of the haptic guide zones is activated,haptic forces associated with the activated zone guide the tool untilthe assigned task has been completed, the target end point has beenreached, or the haptic guide zone has been deactivated by the surgeon.This process may be repeated until each of the tasks associated with thehaptic guide zones have been completed and the corresponding virtualhaptic geometries have been closed.

CAS systems 200 configured in accordance with the presently disclosedembodiments may have several advantages. For example, by providing asolution for selectively and/or sequentially activating haptic guidezones, problems associated with interference between adjacentoverlapping haptic boundaries may be significantly reduced. Furthermore,because the presently disclosed embodiments provide a solution forautomatically activating (and deactivating) haptic guide zones based onthe position of cutting tool 103, inefficiencies associated withavoiding regions of overlapping haptic boundaries may be minimized oreliminated.

The foregoing descriptions have been presented for purposes ofillustration and description. They are not exhaustive and do not limitthe disclosed embodiments to the precise form disclosed. Modificationsand variations are possible in light of the above teachings or may beacquired from practicing the disclosed embodiments. For example, thedescribed implementation includes software, but the disclosedembodiments may be implemented as a combination of hardware and softwareor in firmware. Examples of hardware include computing or processingsystems, including personal computers, servers, laptops, mainframes,micro-processors, and the like. Additionally, although disclosed aspectsare described as being stored in a memory one skilled in the art willappreciate that these aspects can also be stored on other types ofcomputer-readable storage devices, such as secondary storage devices,like hard disks, floppy disks, a CD-ROM, USB media, DVD, or other formsof RAM or ROM.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed systems andassociated methods for selectively activating haptic guide zones withoutdeparting from the scope of the present disclosure. Other embodiments ofthe present disclosure will be apparent to those skilled in the art fromconsideration of the specification and practice of the presentdisclosure. It is intended that the specification and examples beconsidered as exemplary only, with a true scope of the presentdisclosure being indicated by the following claims and theirequivalents.

What is claimed is:
 1. A method for selectively activating a virtualgeometry, comprising: establishing a plurality of virtual geometries,each virtual geometry having a target feature and being available to beactivated to guide an instrument to the target feature by restrictingmovement of the instrument within the confines of the virtual geometry;displaying the plurality of virtual geometries on a display to a user;receiving user input selecting a virtual geometry from the displayedplurality of virtual geometries; and activating the selected virtualgeometry; wherein two or more of the plurality of available geometriesoverlap at an overlapping region, and wherein the selective activationof the virtual geometry based on the user input prevents forcesassociated with a first virtual geometry from interfering with forcesassociated with a second virtual geometry while the instrument is in theoverlapping region.
 2. The method of claim 1, the method furthercomprising tracking a position of a reference point of the instrument;wherein receiving user input selecting a virtual geometry from thedisplayed plurality of virtual geometries comprises receiving user inputpositioning the reference point of the instrument within a thresholddistance of a target feature; and wherein activating the selectedvirtual geometry comprises activing the virtual geometry associated withthe target feature.
 3. The method of claim 2, wherein when theinstrument is positioned within threshold distances of two or moretarget features, activating the selected virtual geometry comprises:determining a distance between the reference point and each of the twoor more target features; and activating the virtual geometry having theclosest target feature.
 4. The method of claim 1, wherein the forcesassociated with the first virtual geometry and the forces associatedwith the second virtual geometry provide haptic feedback to the user. 5.The method of claim 1, wherein the plurality of virtual geometries arevirtual haptic geometries configured to provide haptic feedback to theuser.
 6. The method of claim 1, further comprising: automaticallyactivating a virtual geometry based on a tracked position of a referencepoint of the instrument; receiving user input indicating a preference todeactivate the automatically activated virtual geometry; and in responseto the user input indicating the preference to deactivate the virtualgeometry, deactivating the automatically activated virtual geometry. 7.The method of claim 6, wherein each of the plurality of virtualgeometries is associated with a surgical step to be completed, andwherein the deactivated virtual geometry can be reactivated after thesurgical step associated with the virtual geometry activated based onthe user input is completed.
 8. The method of claim 1, furthercomprising: automatically activating a virtual geometry based on atracked position of a reference point of the instrument; wherein theselective activation of a virtual geometry based on the user inputoverrides the automatic activation of the virtual geometry.
 9. Themethod of claim 1, further comprising: tracking a position of areference point relative to the target feature of the selectivelyactivated virtual geometry; and after determining that the referencepoint has reached the target feature, deactivating the selectivelyactivated virtual geometry.
 10. The method of claim 1, furthercomprising: tracking a position of a reference point relative to thetarget feature of the selectively activated virtual geometry; and afterdetermining that the reference point has reached the target feature andthat the reference point is not within a threshold distance of anengagement point, deactivating the selectively activated virtualgeometry.
 11. A computer-assisted surgery system, comprising: aprocessing circuit configured to establish a plurality of virtualgeometries, each virtual geometry having a target feature and beingavailable to be activated to guide an instrument to the target featureby restricting movement of the instrument within the confines of thevirtual geometry; a display configured to display the plurality ofvirtual geometries to a user; an input/output device configured toreceive user input; and a tracking system configured to track a positionof a reference point of the instrument; wherein in response to theinput/output device receiving user input selecting a virtual geometryfrom the displayed plurality of virtual geometries, the processingcircuit is further configured to activate the selected virtual geometry;and wherein two or more of the plurality of available geometries overlapat an overlapping region, and wherein the selective activation of thevirtual geometry based on the use input prevents forces associated witha first virtual geometry from interfering with forces associated with asecond virtual geometry while the instrument is in the overlappingregion.
 12. The computer-assisted surgery system of claim 11, wherein inresponse to the input/output device receiving user input positioning thereference point of the instrument within a threshold distance of atarget feature, the processing circuit is configured to activate thevirtual geometry associated with the target feature.
 13. Thecomputer-assisted surgery system of claim 12, wherein when theinstrument is positioned within threshold distances of two or moretarget features, the processing circuit is configured to: determine adistance between the reference point and each of the two or more targetfeatures; and activate the virtual geometry having closest targetfeature.
 14. The computer-assisted surgery system of claim 11, whereinthe forces associated with the first virtual geometry and the forcesassociated with the second virtual geometry provide haptic feedback tothe user.
 15. The computer-assisted surgery system of claim 11, whereinthe plurality of virtual geometries are virtual haptic geometriesconfigured to provide haptic feedback to the user.
 16. Thecomputer-assisted surgery system of claim 11, wherein the processingcircuit is further configured to: automatically activate a virtualgeometry based on the tracked position of the reference point; inresponse to the input/output device receiving user input indicating apreference to deactivate the automatically activated virtual geometry,deactivate the automatically activated virtual geometry.
 17. Thecomputer-assisted surgery system of claim 16, wherein each of theplurality of virtual geometries is associated with a surgical step to becompleted, and wherein the deactivated virtual geometry can bereactivated after the surgical step associated with the virtual geometryactivated based on the user input is completed.
 18. Thecomputer-assisted surgery system of claim 11, wherein the processingcircuit is further configured to: automatically activate a virtualgeometry based on the tracked position of the reference point; whereinthe selective activation of a virtual geometry based on the user inputoverrides the automatic activation of the virtual geometry.
 19. Thecomputer-assisted surgery system of claim 11: wherein the trackingsystem is further configured to track the position of the referencepoint relative to the target feature of the selectively activatedvirtual geometry; and wherein the processing circuit is furtherconfigured to deactivate the selectively activated virtual geometry inresponse to determining that the reference point has reached the targetfeature.
 20. The computer-assisted surgery system of claim 11: whereinthe tracking system is further configured to track the position of thereference point relative to the target feature of the selectivelyactivated virtual geometry; and wherein the processing circuit isfurther configured to deactivate the selectively activated virtualgeometry in response to determining that the reference point has reachedthe target feature and that the reference point is not within athreshold distance of an engagement point.